Strong, Flexible Composite Combines Minerals, Thermoplastic

Materials makers are coming up with a variety of ways to make structural composites, from recycled plastic bottles that become weight-bearing elements of heavy load bridges to substituting coconut fibers for traditional ceramic fibers in biocomposite tiles for walls and floors. Now a thermoplastic-mineral composite replicates wood's fibrous structure and is contending with wood, wood plastic composites, plastics, and metal in a range of structural applications.

The material is a fully fiberized, molecularly oriented, lineal composite system from Eovations, which is licensing the technology, said Claude Brown, vice president of technology & innovation for Eovations, in an interview. At present, structural applications include marine uses for docks, pier, seawalls, and pontoons, as well as the cargo decks of vehicles and truck liners. One of the company's primary near-term focuses is the building industry, where it can be used as a Trex replacement for structural framing, as well as for non-structural uses.

Eovations's mineral/thermoplastic composite has a modulus of elasticity between 500,000 psi and 800,000 psi, but a modulus of rupture of zero. It can flex extremely well, but does not have a catastrophic failure mode. (Source: Eovations)

But it's a bit different from other wood and polymer composites like Trex. The company uses a proprietary extrusion/drawing process to combine mineral particles with a thermoplastic matrix. It's a composite because it combines two distinct materials and phases, Brown told us. Half of the composite consists of polymer, mostly polypropylene, and the other 50 percent is a mineral filler. The process first creates a small-scale fiber matrix, and then orients polymer chains within the individual fibers into a composite structure.

Eovations uses several mineral fillers. These are present primarily as a physical nucleation site, not a chemical nucleation site, since their chemistry is less important than the right size and aspect ratio, Brown said. The material also replicates the structure of wood with an internal closed cell structure that reduces the weight of the material without sacrificing its strength. Open spaces are included as capillary-like structures between the strands, so that about 56 percent of the composite's volume is air.

The new composite has a much higher modulus of elasticity at 500,000 psi to 800,000 psi than Trex, for example, which is typically 300,000 psi, and it costs about the same as similar synthetic decking products for construction uses. It does not have a catastrophic failure mode: the modulus of rupture is zero. That means it can be valuable as a hurricane-proof material, said Brown.

"A piece of it 5/16 inches thick can pass all the hurricane standards by itself, not including any other structure. A 55 mph wind leaves only a small impression, about 3/8 inches, but it bounces off of the structure," he told us. The composite's moisture absorption is less than 0.2 percent and densities are between 0.5g/cc and 1.0 g/cc (you can access the technical data sheet here).

The material's wide processing window means that properties of the finished composite, such as density, can be customized. It can be worked like wood with common woodworking tools, including cutting, sawing, machining, and drilling. Conventional fasteners such as nails, screws, and staples can be used, and their acceptance and hold is even better than wood's.

The technology was originally developed at Dow Chemical, but never commercialized, according to Brown. Eovations's parent company, Universal Forest Products, bought the technology from Dow in 2010.

@Ann: Okay, apparently you learned some highly non-standard definitions. According to Instron's glossary of materials testing, modulus of rupture is synonymous with flexural strength. If you Google "modulus of rupture," you'll see that this is the most commonly used definition. Clearly, "the flexural strength is zero" would not be a correct statement.

The derivation of the most widely-used modulus of rupture formula assumes that the material deforms in a linear and elastic way, so the formula no longer applies after the yield point. However, this doesn't mean that "the modulus of rupture is zero"; it just means that the formula no longer correctly predicts the stress.

As far as "catastrophic failure" goes, the most common definition seems to be "(sudden) failure from which recovery is impossible."

Give me any material, and I guarantee I can find a way to make it fail such that you'll never get it back into its original shape. Hit it hard enough, and it will fail catastrophically.

@Ann: The fact that something bends before it breaks doesn't mean that it doesn't have a catastrophic failure mode -- it just means that it bends first. If you continue to load it, it will fail catastrophically, sooner or later. (Steel bends before it breaks, too, but you can point to any number of catastrophic failures of steel structures).

And "the modulus of rupture is zero" is just plain wrong. As I said, modulus of rupture is a measure of the load-carrying capacity of a beam. If the load-carrying capacity were zero, it would be useless.

Modulus of rupture just isn't a useful number for something that yields before it breaks, since the equation assumes no yielding.

TJ, thanks for the clarification. Determining that would probably take a detailed lifecycle analysis. Although these are now voluntary, and not nearly as common as many of us would like, I hope that someday they'll be required as an item on the data sheet.

The original question was worse for the environment - structural members made from wood or the composites described in the article.

The lumber industry sells itself as renewable because the companies plant trees to replace the ones harvested, and the renewal process is pretty straight forward - Cut down a forest, make lumber, plant seedlings in the now-cleared area, come back in 20 years or so and repeat.

IF the composites are made from organic feedstocks, they may be the equal of wood's impact on the environment. If they're made from petrochemicals, one would probably have to say they have a larger impact.

Lou, I thought the hurricane-resistance use was one of the most intriguing and unique about this material, aside from the more mundane uses like deck replacement. Chuck, this is not aimed at high load-bearing structural components such as bridge beams: as we state, it's for lighter construction uses such as decks and pontoons.

Interesting development. Nice to see these sort of efforts moving forward. They may not always be the most cost effective, but it's a step in the right direction. I wonder if this is one of those "we can do it in the lab, so let's see if we can sell it" sort of things or if they actually had a specific market in mind.

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